This invention relates generally to flexible containers for transporting and storing goods and, more particularly, to such flexible containers which are constructed from a tubular blank formed so as to include a shell, a bottom and a filling aperture, the container being liftable by its upper end and wherein pleats are formed in the container shell having lower parts which are folded into the plane of the container bottom, the free lower margin of the tubular blank being closed by a bottom seam located substantially in the central region of the container bottom, and upper and lower planes of each pleat both folded down into the plane of the container bottom and affixed to one another by a connecting seam.
A first type of flexible transport and storage container for bulk goods is known and is disclosed in Finnish Pat. No. 61,006. A container of this type is preferably provided with an inner bag of impermeable material and substantially formed of a single piece of tight material having lifting loops in its upper portion and a filling aperture in a central portion. The bottom portion of the container includes at least four flaps provided in respective pairs of equal size flaps and which constitute linear extensions of the container shell, the flaps of each pair being joined at the lower margin so as to form juncture lines which intersect at a single point. Similar bags of this type are also disclosed in CH-Pat. No. 362,970 and in German Publicizing Print No. 1,126,795. A drawback inherent in large-size bags of this first prior art type is that the bottom of the bag cannot withstand the high stresses to which the central region of the bag bottom is subjected when the bag is filled with bulk goods. This inability to withstand high stresses is due to the fact that the juncture lines provided in the bag bottom intersect at the central point of the bag where the highest peak stresses occur. The peak stresses occur in the central region of the bag bottom as a result of the fact that when filled with bulk goods, the bottom portion of the large-size bag tends to assume a rounded shape.
A second type of prior art flexible container for transporting and storing bulk goods is disclosed in Finnish Pat. No. 57,382. In a container of this prior art type, the bottom is constructed of strip-like parts which are joined by connecting seams which are disposed to extend at a substantial distance from the central area of the bag bottom. A container bottom of this type is capable of reliably withstanding all stresses which may be imposed on the container bottom in normal practice since no connecting seams are disposed in the central region of the container bottom which would tend to reduce the strength thereof. This known design for a container bottom may be utilized equally as well as containers provided with lifting loops in their upper portion as well as in containers which are not provided with such lifting loops, such containers which are not provided with lifting loops, i.e. so-called open bags, are sealed at their filling apertures and may be lifted, for example, utilizing an appropriate lifting hook.
Containers of the second prior art type discussed above have the drawback that the relatively complex structure of the container bottom prevents the manufacture of such containers in an automated fashion or at least makes such automation exceedingly difficult and costly to accomplish. Moreover, several separate manufacturing steps are required to construct containers of this second prior art type, namely, cutting the bottom strips and the affixation of the same. Such drawbacks result in relatively high production costs.
A third type of prior art large-size bag is also known, namely, the so-called pleated large-size bag. In this construction, the pleats are formed in the shell of the bag having lower parts which are folded into the plane of the container bottom. In such prior art pleated large-size bags, the pleats are open, whereby the lower side of the pleat is free while the upper side of the pleat becomes taut when stress is applied on the bag. The peak stress is particularly directed on the so-called apex of the pleat, which is constituted by the inside apex point of the large-size bag. Prior art containers of this type have the drawback that the same are frequently ruptured due to the stress conditions arising at the pleat apex, the rupture usually initiating at the pleat apex.
In Finnish Pat. No. 61,174, a so-called pleated large size bag is disclosed, in which the free lower margin of the tubular blank is closed by a bottom seam which is located substantially in the central region of the bottom and the upper plane of each pleat is affixed to the lower plane thereof, by a connecting seam. Such a connecting seam will result in an increased strength of the container bottom by shifting the stresses from the central region of the bottom to the marginal areas thereof. Generally, the opposite planes of the pleat are affixed to each other by a connecting seam of this kind, so that the seam affixes the upper plane of the pleat to the margin of the lower plane of the pleat.
Although relatively high strength of the container bottom is achieved with the flexible container disclosed in Finnish Pat. No. 61,174, this prior art design is also susceptible to failure or rupture always starting at the apex of the pleat and proceeding from the apex point to the outlying bottom area, weakening the bottom of the container which ultimately fails along the bottom seam